The invention disclosed and claimed herein generally pertains to a method and apparatus for correcting or compensating for distortion of the shape of the X-ray field in an X-ray imaging system. More particularly, the invention pertains to a method of the above type wherein distortion results from X-ray beam angulation, that is, projection of the X-ray beam toward the X-ray detector at an angle of less than 90 degrees. Even more particularly, the invention pertains to a method of the above type wherein the geometric center of the X-ray beam field, as projected into the plane of the detector and distorted by the angulation, is computed and selectively positioned with respect to the detector.
As is well known, in a typical X-ray imaging system a patient is positioned between an X-ray tube and an image receptor having a planar imaging surface, such as an X-ray film or a digital solid state detector. The tube projects a beam of X-radiation toward the detector surface and through body structure of the patient which is to be imaged. The area of projected X-radiation which is incident on the detector defines the active imaging area (AIA) . Generally, the X-ray beam field or field of view (FOV), which is defined herein to be the intersection of the projected beam and the detector plane, must be coincident with, or lie within, the boundaries of the detector surface in order to avoid loss of image data. The FOV may be adjusted by rotating or tilting the tube to vary the direction of the projected X-ray beam, and also by operating a collimator to vary the width and length dimensions of the X-ray beam. Further adjustments may be made by linear translation of the tube and/or the detector
If the tube is oriented so that the X-ray beam, or more particularly the beam axis or central ray thereof, is directed in perpendicular or orthogonal relationship to the detector plane, the beam field projected into the detector plane will be of rectangular configuration. However, an X-ray technician or operator, when setting up for an imaging procedure, may need to angulate the beam, that is, rotate or pivot the X-ray tube so that the beam is directed toward the detector at an angle of less than 90 degrees. This may be necessary, for example, to ensure that the beam passes through the specific body structure of the patient which is to be imaged. As the X-ray beam is increasingly angulated, however, the beam field projected into the detector becomes correspondingly distorted and trapezoidal, and the location of the center of the central ray of the beam becomes offset with respect to the geometric center of the projected X-ray field. As a consequence of these decentering and distorting effects, the task of the operator to achieve alignment becomes more difficult, time consuming and prone to error. More specifically, it may become necessary for the operator to re-collimate the X-ray beam to a smaller field size, or to realign the image detector with the beam field, in order to achieve optimal centering of the projected image onto the detector. It may also be necessary to reposition the patient. In the absence of optimal or appropriate centering, anatomical cutoff may occur during the imaging process, which would necessitate that the examination be repeated, thus contributing to increased procedure cycle time, higher examination costs, and higher net radiation doses to the patient. Moreover, if beam angulation causes the angle of beam incidence to exceed +/xe2x88x9210xc2x0, current regulations prevent use of an automated device to adjust the collimator. Accordingly, the system operator must manually collimate the beam to a desired field size.
The invention relates to an arrangement for enabling an operator of an X-ray system to quickly and efficiently align the X-ray detector with the imaging subject and the projected X-ray beam, in a manner which avoids anatomical cutoff and inaccurate positioning, notwithstanding any angulation of the beam or distortion effects resulting therefrom. Moreover, the invention reduces cycle time by enabling autocollimation for angles of beam incidence in excess of +/xe2x88x9210xc2x0, and by providing for automatic shifting of the detector.
One embodiment of the invention is directed to a method of alignment for an imaging system provided with an X-ray tube and a detector having a surface lying in a specified plane, wherein the tube is spaced apart from the detector by a source-to-image distance (SID) along a first axis, and wherein the tube is disposed to project an X-ray beam characterized by a beam direction angle xcfx86 and a beam width angle xcex3 toward the detector. The method includes the step of specifying the length dimension of the beam field projected into the specified detector plane, the length dimension being measured along a second axis which is orthogonal to the first axis. The method further comprises the steps of computing a value of the beam width angle from the specified beam field length, the SID and the beam direction angle, and then locating the geometric center of the beam field from the computed beam width angle, the SID and the beam direction angle. Thereupon, a selected positional relationship is established between the center of the detector surface and the geometric center of the beam field.
In a preferred embodiment of the invention, the locating step comprises determining the point at which the central axis of the projected beam intersects the detector plane, and then locating the geometric center at a point which is spaced apart from the beam axis intersection point, along the second axis, by a computed offset value. The offset value is computed from the SID and beam direction angle, and also from the computed value of the beam width angle. Preferably also, the center of the detector surface is aligned with the geometric center of the beam field, after the offset value has been determined. In one useful embodiment, the specified length of the beam field is equal to the length of the detector surface extending along the second axis. In another useful embodiment, the specified length of the beam field is selectively less than the length of the detector surface along the second axis.